Etching



United States Patent ETCHING Charles W. Hopkins and John A. Easley, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application September 28, 1956 Serial No. 612,886

Claims. (Cl. 41-43) ing surface, such as a photoengraving, a plate of an acidsoluble metal such as magnesium, zinc, or one of their alloys is firstly coated with a light-sensitive coating or enamel, applied to the generally flat or cylindrical metal surface. This coated surface is then exposed to light through a negative having an image thereon so as to produce a corresponding image on the coating. The exposed coated surface is then "developed, forming an acid-resistant coating in the form of the image produced by the exposure. This acid-resistant coating is further hardened by heating and the image-bearing surface of the plate is then subjected to etching by an acid. In prior art techniques, as the etching proceeds, the acidtends to etch laterally and thereby undercut the resist, causing a weakening or a distorting of the image. While art techniques have proposed to prevent or reduce the amount of undercutting by repeatedly powdering the plate, and partially etching between each powdering application, this has been a time-consuming and difficult operation.

It is a principal object of the present invention to provide an improved method for etching metal and particularly photoengraving plates. Another object of the present invention is the provision of etching baths for carrying out the improved process for etching. Still a further object of the present invention is to provide etching baths containing particular sulfated materials, e. g., sulfated fats, oils and/or waxes. Another object of the present invention is the provision of such etching baths containing particular amounts of the above materials. Other objects will become apparent hereinafter.

This invention is directed to an etching technique which may be accomplished by providing an etching bath containing aqueous nitric acid, an immiscible organic liquid substantially unreactive with the aqueous nitric acid solution, and a water-soluble, anionic, surface-active half ester of sulfuric acid, containing at least one hydrophobic group therein, said hydrophobic group containing between 10 and 26 carbon atoms in an uninterrupted chain, said ester also containing at least 8 percent sulfation, the ester usually being derived from the sulfuric acid treatment of naturally-occurring fats, oils and waxes.

Suitable metals which may be etched by the process and bath of the present invention are magnesium, magnesium base alloys containing at least 85 percent magnesium, and zinc and zinc base alloys containing at least '90 percent zinc. To prepare a photoengraving plate, the

surface of the metal is first coated uniformly with a lightsensitive material, exposed to light through a negative, developed, and, if necessary, heated to increase the acid resistance of the resist. When the plate to be etched has been so treated, it is then ready for use in the method of the present invention.

Aqueous nitric acid for the bath of the present invention is employed in an amount by weight of the total bath between about .lfleasLgimstsenh i fil-ls. concentrations in the range of from 3 to 20 percent HNO being most generally useful, while a concentration of from 4 to 12.5 percent is preferred. The aqueous nitric acid 1s mixed with a watemrfiible organic material which is liquid at those i'mperafthi'e's employeif'for' etching. This organic liquid should also be substantially unreactive with the aqueous nitric acid solution. Suitable organic materials which may be used in the method and bath of the 5125- cut invention include, for example, the various petroleum fractions boiling in the range of to 390-C., such" as gasoline, benzine, kerosene, coal oil, and lubricating oils. Other organic liquids which may be employed include tuipentine, perchloroethylene, the diethylpegggnes,

,..';.Ii? 1e 12 smi hq te rashl ride, etc. The a ngggnt gin; ganic liquid employed will vary with the type of metal sur ace to be processed, but will fall within the range of 1 t o p ercentby weight of the total bath and preferably is from 5 'to 8 percent.

A third corn onent of the bath, e. g., a sulfated fat oil or W a sulfate-radical-containing, organic surface-active agent. This component can further be described as containing at least in part a half ester of sulfuric acid, the component being water-soluble, anionic, and containing at least 8 percent sulfation, preferably at least 14 percent. Percentage of sulfation, as used herein, is intended to mean the average amount of sulfate (80,) contained in the molecule divided by the practical amount of sulfation possible multiplied by one hundred. The hydrophobic portion of the surface-active agent must contain at least one aliphatic organic group containing at least 10 carbon atoms and not more than 26 carbon atoms, and preferably from 14 to 26 carbon atoms. Suitable sulfated materials which may be used in the bath of the present invention include, for example, sulphated tall oil, sulfated castor oil, sulfated sperm oil, sulfated tallow, sulfated propyloleate, alkylarylpolyether sulfates, sulfated amides, sulfated alcohols, the monoglyceride ester of coconut oil sulfate, etc. These materials are generally available commercially under various trade names. It has been found that the amount of the third component employed is critical, the effective range of concentration being between 0.4 and 2.0 percent by weight of a total etching bath. For example, Monosulph (Nopco Chemical Company-a sulfated castor oil having 45 percent sulfation containing 30 percent water, 2 percent sodium sulfate, 24 percent sulfated ester, 28 percent unreacted neutral oil and 16 percent sulfated acids and soap and free fatty acids) has an optimum effective concentration of 1.16 percent. In practicing the present invention it has been found to be desirable to impinge the bath against the surface to be etched, as by splashing. In theory, at least, the etching bath of the present invention forms a partially acid-resistant film on the resist-free metal surfaces, and when the bath is impinged against the surface to be etched in a direction normal to that surface, the film is broken. On the other hand, when the bath hits the sides of the relief the film is generally not broken because of insufficient striking force and etching of the sides (or undercutting) is substantially reduced. It is preferred to employ an etching machine of the type disclosed in Patent No. 2,669,048, issued February 16, 1954, and entitled Etching Machine. In this machine elongated paddles dipping into the bath composition intermittently throw, by splashing, the etching bath composition in sheets upwardly against the image bearing side of the object being etched, e. g. a plate.

As used in this specification, the term etch factor, is defined as the ratio of the depth of the etch adjacent to a line of resist divided by one-half of the loss in width of metal immediately beneath the resist. Etch factors of 10 and higher are readily obtained by the method of the present invention and etch factors as high as 100 are not unusual.

Each of the baths was relatively stable and each bath gave a very favorable etch when conditions were adiusted.

Various modifications may be made in the present in- The following examples are given to illustrate the pres- 5 vention without departing from the spirit or scope thereent invention, but are not to be construed as limiting. of, and it is to be understood that we limit ourselves only as defined in the appended claims. EXAMPLE 1 u We claim:

A magnesium phetflengiaving sheet having a pommal 1. The method of etching the surface of an acidc011119095011 f 3 Percent aluminum, Percent 2111C, and 10 soluble metal selected from the group consisting of magminol alloying e e ents and impurit e balance nesium, zinc, magnesium-base and zinc-base alloys said nesium, was photoprinted and developed in the convensu fa having portions thereof masked with an acid fiohal manner to P p an acid resistant Coating in h resistant coating, which includes: repeatedly impinging form of an image- A machine of the yp described upon the surface of the object so as to contact both the U. S. Patent No. 2,669,048, was provided and approxil masked and unmasked portions thereof an aqueous solumately 3 liters of water at a temperature of about 601 ti hi h c ntain (1) from 1 t 25 percent of HNO,, F. placed therein. Thereafter, nitric acid of 42 liaume (2) a water-immiscible organic liquid substantially untechnical was added to make a final concentration O reactive with the aqueous nitric acid, and (3) between 0.4 a in the bath of Percent Next, Shfhclent MOHO- and 2.0 percent of a component which includes a watersulph P Chemia1C0mPanY-a 45 p n sulfifted soluble, anionic, sulfate-radical-containing, surface-active castor oil) to make a 1.22 percent final solution mixed agent containing at l t one h d hobi group of with sufficient diethyl-benzene to get a final concentration f m 10 to 26 ca b n atoms in an uninterrupted chain of 7 percent was added to the bath, and the volume and said agent containing at least 8 percent sulfation. adjusted with water to 5700 milliliters. The solution was 2, The method of etching the surface of an acid.. mixed y running the Paddles r {W0 f A P soluble metal selected from the group consisting of mageng'raving Sheet, as above described, havlng h image nesium, zinc, magnesium-base and zinc-base alloys, said thereon was descummed with an aqueous solution consurface having portions thereof mask d ith an acid-retaining 12 ounces of potassium hydroxide and 4 ounces sistant coating, which includes: repeatedly impinging up- Of Potassium p nganate per gallon of solution. This on the surface of the object so as to contact both the plate was then placed in the plate holder of the mamasked and unmasked portions thereof an aqueous soluchine, and the lid closed. tion which contains (1) from 1 to 25 percent HNO,,

After the temperature of the bath had been brought (2) a water-immiscible organic liquid substantially unto 72 F. by the heat exchanger, the paddles were reactive with the aqueous nitric acid, and (3) between started and adjusted to 500 R. P. M. and the etching con- 0.4 and 2.0 percent of a water-soluble, anionic, surfacetinued for 8 minutes. There was thus obtained an aver- 30 active, sulfated, naturally-occurring oil containing at least age etch depth of .0l4-.0l5 inch with a smooth etched 8 percent sulfation. surface and a sharply delineated relief. The etch factor 3. The method of claim 2 wherein the sulfated oil is was 50. castor oil containing about percent sulfation.

EXAMPLE 2 4o 4. The method of claim 2 wherein the organic oil is diethylbenzene.

In a mannlr slmllar to a of the f Exampl? 5. The method of etching the surface of a magnesium except that zmc photoengraving plate having a nominal plate having a portion of its surface masked with an composition of 0.7 percent aluminum, and minor alloying acid-resistant coating, which includes. repeatedly 1melements, balance zmc, was substituted for the magnesium l t th amount of Monosul h was reduced to 104 pinging upon the surface of the plate so as to contact p a e p 45 both the masked and unmasked portions thereof an percent, the paddle speed was 600 R. P. M., and the aqueous solution which contains (1) 4 to 12.5 percent time was 9% minutes. There was thus obtained an avernitric acid, (2) 5 to 8 percent diethylbenzene, (3) and age etch deph of P havmg.a smooth etched 1.16 percent sulfated castor oil having 45 percent sulfaface and a sharply delineated relief. The etch factor tion was EXAMPLE 3 6. An etching bath comprising an aqueous mixture of l to 25 percent HNO from 1 to 12 percent of a water- In a manner similar to that'of the foregoing Example immiscible organic liquid substantially unreactive with 1, other half esters of sulfuric acid were substituted for the aqueous nitric acid, and between 0.4 and 2.0 perthe above Monosulph and the etching procedure repeated. cent of a water-soluble, anionic, sulfate-radical-containing The following table summarizes these results: surface-active agent containing at least one hydrophobic Table Etch Ooncentra- Percent Trade name Sulfated Principal Combined and Uncombtned Manufacturer Factor tion Per- Optimum Fatty Acid Content Range cent Range Concentra- Tested tlon Turkey Red 011.--- Castor 0il. RicinoleicAcid Nopco Chem. Co... 2.6-100+ 0.1-1.8 1.0

Cordon 200/ do ....-in Ftnetexlnc 4.5-25 0.30-1.04 0.90

Cordon900 Sperm Oil. Stearic. Oieie, Linoleic, Palmitoleic, .-.-do 2.5-15 0.18-1.17 1.0

Arachldlc, Palmltlc, Myristic, Myrlstoleic, Laurie, and Behenic Acids.

cot-don LB8I0 TailOils 50-60% City-C1D Fatty Acids (primarily -....do 6.2-15 0.30-0.31 0.7

Cit-Cit) with 34-40% rosin acids.

DrutoneD Coconut and Olive Laurie,Myristlc.Paimltic,Ole1c,Linoleic, E. F. DrewdtCo 6-8 0.25-1.17 1.1

Oils. and Stearie Acids.

Estol-T75 Tallow Myristic. Palmitic, Stearlc, Oleic and ....do 3-14 0.25-1.17 1.1

Llnolelc Acids.

In the above table, the Turkey red oil contained approximately 22 percent sulfation, while the others contained between 10 and 20 percent sulfation, depending on the amount of reaction products remaining with the desired material after purification by the supplier.

group of from 10 to 26 carbon atoms in an uninterrupted chain, said agent containing at least 8 percent sulfation.

7. An etching bath comprising a mixture of 1 to 25 percent nitric acid, a water-immiscible organic liquid sub- References Cited in the file of this patent UNITED STATES PATENTS Easley et a1. June 2, 1953 OTHER REFERENCES Schwartz-Perry: Surface Active Agents, publ. 1949, Interscience Pub. Inc., N. Y., p. 462, lines 7-11. 

1. THE METHOD OF ETCHING THE SURFACE OF AN ACIDSOLUBLE METAL SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM, ZINC, MAGNESIUM-BASE AND ZINC-BASE ALLOYS SAID SURFACE HAVING PORTIONS THEREOF MASKED WITH AN ACID RESISTANT COATING, WHICH INCLUDES: REPEATEDLY IMPINGING UPON THE SURFACE OF THE OBJECT SO AS TO CONTACT BOTH THE MASKED AND UNMASKED PORTIONS THEREOF AN AQUEOUS SOLUTION WHICH CONTAINS (1) FROM 1 TO 25 PERCENT OF HNO3, (2) A WATER-IMMISCIBLE ORGANIC LIQUID SUBSTANTIALLY UNREACTIVE WITH THE AQUEOUS NITRIC ACID, AND (3) BETWEEN 0.4 AND 2.0 PERCENT OF A COMPONENT WHICH INCLUDES A WATERSOLUBLE, ANIONIC, SULFATE-RAFICAL-CONTAINING, SURFACE-ACTIVE AGENT CONTAINING AT LEAST ONE HYDROPHOBIC GROUP OF FROM 10 TO 26 CARBON ATOMS IN AN UNINTERRUPTED CHAIN AND SAID AGENT CONTAINING AT LEAST 8 PERCENT SULFATION. 